1. Field of the Invention
The present invention relates to a voltage-controlled oscillator and, particularly, to a voltage-controlled oscillator having a ring oscillator, which is constituted with a plurality of differential amplifiers each having a load circuit.
2. Description of the Related Art
In a recent phase-locked loop (PLL) for a disk servo system of CD-ROM, etc., an output frequency range thereof, which is up to several to ten times a reference oscillation frequency of the PLL, is required. It has been known that the PLL includes a voltage-controlled oscillator whose output frequency is varied correspondingly to an input voltage thereto.
Since the recent PLL has an on-chip structure due to the recent tendency of increase of the scale of integrated circuit and the operation speed thereof, the voltage-controlled oscillator of such PLL should have a structure suitable to be integrated on a chip. That is, it has been required to develop a voltage-controlled oscillator, which is stable regardless of variation of fabrication process, does not require regulation after fabrication and is durable against noise generated in an integrated circuit.
In order to realize such voltage-controlled oscillator, a ring oscillator composed of a plurality of differential amplifiers each having a load, by which the ability of the ring oscillator can be controlled, has been proposed in, for example, IEEE Journal of Solid-State Circuits, Vol. 25, No. 6, November, 1990, pp. 1385 to 1394, IEEE Journal of Solid-State Circuits, Vol. 27, No. 11, November, 1992, pp. 1599 to 1607 and U.S. Pat. No. 5,412,349. The ring oscillator, proposed in each of these articles has characteristics suitable to be integrated on a chip, since the ring oscillator is composed of the differential amplifiers whose sensitivity to power source noise is low, the operating point variation between sampled operating points is small due to simultaneous feedback control of the operating points and the operating point variation due to relative variation within a chip can be made small because an input impedance of the load is low.
These prior arts will be described in more detail with reference to FIGS. 13 to 18. The prior art voltage-controlled oscillator is shown in FIG. 13. In FIG. 13, the voltage-controlled oscillator 103 comprises a ring oscillator 101 and a ring oscillator control circuit 102 for controlling an operation of the ring oscillator.
FIG. 14 is a block circuit diagram of the ring oscillator 101, which is constructed with differential amplifiers 105 to 108 having identical circuit constructions. Each differential amplifier includes a plus input I1, a minus input I2, a plus output O1, a minus output O2, a current control voltage terminal IC and a load control voltage terminal CL. The differential amplifiers 105 to 108 are connected in series and an output of the differential amplifier 108 is fed back in reverse phase to an input of the differential amplifier 105 to constitute a 4-stage ring oscillator 104. Since the ring oscillator 104 is composed of the differential amplifiers 105 to 108, the sensitivity thereof to power source noise is restricted due to its high ability of removing the power source voltage variation. The current control voltage terminal IC of each differential amplifier has a function of controlling a circuit current thereof and the load control voltage terminal CL thereof has a function of controlling an in-phase output voltage such that the in-phase output voltage is always equal to the reference voltage shown in FIG. 13 by regulating the performance of the load of the differential amplifier.
FIG. 15 is a circuit diagram of the ring oscillator control circuit 102 shown in FIG. 13. In FIG. 15, an NMOSFET 111 has a gate connected to an oscillation frequency control voltage terminal, which is an input terminal of the voltage-controlled oscillator 103, a source connected to a resistor 110 and a drain connected to a drain of a PMOSFET 112. The gate of the PMOSFET 112 is connected to the drain thereof and a source of the PMOSFET 112 is connected to a power source terminal 115. A differential amplifier 113 has an identical construction to that of each of the differential amplifiers 105 to 108, and input terminals I1 and I2 are commonly connected to the reference voltage terminal and output terminals O1 and O2 are commonly connected to a plus input terminal of a single-end operation amplifier 114. An output terminal of the single-end operation amplifier 114 is connected to a load control voltage terminal CL of the differential amplifier 113. The drain of the PMOSFET 112 is connected to a current control voltage terminal IC of the differential amplifier 113. A minus input terminal of the operation amplifier 114 is connected to the reference voltage source. The operation amplifier 114 controls the voltage of the load control voltage terminal CL of the differential amplifier 113 such that the output voltage of the differential amplifier 113 becomes equal to the reference voltage. Since the differential amplifier 113 has the same construction as that of any of the differential amplifiers 105 to 108, which constitute the ring oscillator 101, and the circuit current and the load control voltage thereof are also the same as those of any one of the differential amplifiers 105 to 108, its in-phase output voltage becomes equal to that of the differential amplifiers 105 to 108.
FIG. 16 is a circuit diagram of one (121) of the identical differential amplifiers 105 to 108 and 113. In FIG. 16, the differential amplifier 121 includes an input differential pair of NMOSFET""s 124 and 125 and an NMOSFET 126 as a current source. Load circuits 122 and 123 have identical circuit constructions each shown in FIG. 17 or 18. The load circuit shown in FIG. 17 is composed of an NMOSFET 132 having a gate supplied with a load control voltage and an NMOSFET 133 having a gate and a drain connected to the gate thereof. A terminal 131 is connected to one of the output terminals O1 and O2 shown in FIG. 16. Since the gate of the NMOSFET 133 is connected to an output terminal 131, an impedance looked from the terminal 131 of the NMOSFET 133, that is, an output impedance, is in inverse proportion to a mutual conductance of the NMOSFET 133 and has a low value. Further, since the NMOSFET 132 having the gate supplied with the load control voltage operates in a saturation region, when the load control voltage is low and the following relation is established between the voltage Vo at the output terminal 131 and the load control voltage VCl:
VCLxe2x88x92Vth less than Vo
where Vth is the threshold voltage of the NMOSFET 132. Therefore, the output impedance of the load becomes very large. That is, the output impedance is represented by xcex94V/ xcex94i, a ratio of voltage change xcex94V to current change xcex94i. In the saturation region, since current i is substantially constant, xcex94i is very close to 0 and the output impedance becomes substantially infinite. However, in order to make the in-phase output voltage constant even when the operating current is changed, the NMOSFET 132 whose performance can be controlled by the load control voltage is indispensable. That is, it is impossible to control the in-phase output voltage by only the NMOSFET 133 since the in-phase output voltage is varied by the operating current. Consequently, it becomes possible by using both the NMOSFET""s 132 and 133 to reduce the output impedance of the load to a low value and to make the in-phase output voltage constant.
The load circuit shown in FIG. 18 includes, in addition to the NMOSFET""s 132 and 133 shown in FIG. 17, an NMOSFET 134 having a gate supplied with the load control voltage and provided on the source side of the NMOSFET 133. In the load circuit shown in FIG. 17, when the operating current is reduced and a substantial portion of the operating current flows through the NMOSFET 133, an amplitude of the oscillation frequency output is reduced in a range of operating current smaller than the operating current. In the load circuit shown in FIG. 18, such phenomenon does not occur due to the provision of the NMOSFET 134.
In this case, however, since an output impedance of the series circuit including the NMOSFET""s 133 and 134 is determined by the NMOSFET 134, the output impedance of the series-connected NMOSFET""s 133 and 134 becomes very large when the NMOSFET 134 operates in the saturation region. As a result, there is a lower limit of the output frequency range of the voltage-controlled oscillator 103 constituted with the differential amplifiers 121.
Now, the reason for the necessity of maintaining the in-phase output voltage at constant will be described. In order to connect the voltage-controlled oscillator to a usual CMOS digital circuit, it is necessary to convert the differential output of the ring oscillator 101, that is, the voltage-controlled oscillator 103, into a single-ended CMOS level. In order to realize such conversion, it is necessary to stabilize the in-phase output voltage of the differential output of the voltage-controlled oscillator 103, otherwise, a circuit for converting the differential output into the single-ended CMOS level does not operate normally or, if it operates normally, the operating speed and/or the duty cycle may be degraded.
The purpose of reduction of the output impedances of the loads is to prevent the operating points of the differential amplifiers 105 to 108 from being substantially varied due to relative variation between the current source transistors 126 of the differential amplifiers 105 to 108 and 113 and transistors constituting the load circuits and to restrict the influence of coupling through other wiring and/or a silicon substrate of the integrated circuit.
As mentioned, although the voltage-controlled oscillator including the ring oscillator constituted with the differential amplifiers 121 each having the load circuit shown in FIG. 17 or 18 has characteristics suitable for circuit integration, the range of oscillation frequency thereof is narrow. Therefore, it is difficult to use the prior art voltage-controlled oscillator for the PLL of the disk servo system of such as CD-ROM, which requires the output frequency range from several to ten times the reference frequency.
Further, although the load circuit shown in FIG. 17 or 18 includes a transistor having a gate short-circuited to a drain thereof, the in-phase output voltage of the differential amplifier must be high enough when the threshold value of the transistor is high. However, when the in-phase output voltage is high, the paired differential transistors 124 and 125 are not saturated, so that it becomes impossible to obtain a voltage gain. Therefore, it is necessary to sufficiently reduce the in-phase output voltage of the differential amplifier 121. Accordingly, special fabrication steps dedicated thereto must be added in order to reduce the threshold voltage of the transistors constituting the load circuit.
An object of the present invention is to provide a voltage-controlled oscillator having a wide oscillation output frequency range, which can be fabricated without adding any special fabrication step to a usual fabrication method of a voltage-controlled oscillator.
In order to achieve the above object, a voltage-controlled oscillator according to a first aspect of the present invention includes a ring oscillator constituted with a plurality of differential amplifiers each having a first input terminal, a second input terminal, a first output terminal, a second output terminal and a current limiting terminal, the first and second input terminals of the differential amplifiers being ring-connected to the corresponding first and second output terminals of the differential amplifiers, a first load circuit connected to each of the first output terminal, a second load circuit connected to each second output terminal and is featured by that the first and second load circuits always operate in a linear region.
A voltage-controlled oscillator according to a second aspect of the present invention includes a ring oscillator constituted with a plurality of differential amplifiers each having a first input terminal, a second input terminal, a first output terminal, a second output terminal and a current limiting terminal, the first and second input terminals of the differential amplifiers being ring-connected to the corresponding first and second output terminals of the differential amplifiers, a first load circuit connected to each of the first output terminal and supplied with a first and second load control voltages, a second load circuit connected to each second output terminal and supplied with the first and second load control voltages and is featured by that the first and second load circuits always operate in a linear region according to the first and second load control voltages.
With such circuit construction of the voltage-controlled oscillator, an output impedance of the differential amplifies is always restricted to a low level, so that it is possible to widen the oscillation frequency thereof.